The present invention relates generally to capturing images with a digital camera, and more particularly, to a system for controlling overexposure of images produced by a digital camera.
Correct exposure is of fundamental importance in capturing digital images. Current digital imaging systems, such as digital single-lens reflex cameras, known as digital-SLRs or DSLRs, contain advanced digital image sensors and processors intended to ensure correct exposure. The digital image sensors used today are capable of capturing and producing images with low signal to noise ratio and a high dynamic range. On-board processors utilize use advanced algorithms to process the digital images received on the image sensor. However, even with advanced technology, achieving correct exposure is not always successful for many reasons.
Artistically, a “correct” exposure is a subjective matter and can depend on the photographer's intent and expectations of the image to be captured and photograph to be created. For example, consider taking a portrait photograph of a person standing indoors, in front of a bright window and outdoor scene. Since the human eye is capable of capturing a wider dynamic range of light than even the most advanced DSLRs, the photographer has to decide if they are going to expose for the person, where their face would be viewable, or expose for the bright scene that is viewable through the window. If the photographer sets their exposure for the person, the bright scene through the window will be overexposed and anything that would be visible through the window to the human eye shows as white in the photograph. In the case where the photographer sets their exposure for the bright scene through the window, the person's face is very dark and potentially unrecognizable in the photograph, since the DSLR does not have the ability to capture the wide dynamic range of the light between the person's face and the bright outdoor scene.
As can be appreciated, photographic compositions often contain both bright and dark regions, where the difference between those regions exceeds the maximum dynamic range of the DSLR's image sensor. In such situations, there exists no one exposure which could be considered “correct” for all possible desired artistic outcomes. Most DSLRs have a built-in exposure metering system that is used to measure the amount of light in a composition to be photographed. Once the amount of light in the composition is determined by the exposure meter, the shutter speed, aperture, and ISO settings may be set to give the best picture brightness or exposure.
The exposure metering system of most DSLRs is operable in different modes. Typical metering modes include: “evaluative” or “matrix” metering mode, “center-weighted” metering, and “spot” metering. In spot metering, measurements are made from a spot in the viewfinder of the camera about the size of a focusing bracket or point. The highlighted focus area, or spot, becomes the active spot for calculating exposure so everything inside the spot is averaged for exposure determination. The photographer typically uses a multi-selector button to move a focal point to different positions in the viewfinder. This allows the photographer to specify which region of the composition is to be used for metering to help ensure that the area of main interest will be properly exposed. However, for all but static shooting situations (e.g., studio shoots) there is not enough time to permit the photographer to perform spot metering, as the subject is moving, the lighting in the scene is changing, or both.
In center-weighted metering, the meter concentrates between 60 to 80 percent of exposure sensitivity towards the central part of the viewfinder. The balance of the metering calculation is then “feathered” out towards the edges of the image to be captured. Thus, in center-weighted metering, exposure is less influenced by small areas that vary greatly in brightness at the edges of the viewfinder, as many subjects are in the central part of the frame.
In evaluative or matrix metering mode, information is gathered from red, green, and blue sensors and factors in distance information provided by the DSLR's lens as it evaluates proper exposure calculation. This metering method analyzes a scene's overall brightness, contrast, and other lighting characteristics. Some matrix metering methods can then compare the scene received by the camera against an onboard database of potentially thousands of images, to determine what the exposure should be.
For example, the matrix metering system evaluates the relative brightness values for several regions in a composition together with the focus distance. The system then concludes that the photographer is shooting a portrait on the beach, where the subject is backlit against a large expanse of white sand. The system then sets an exposure to allow the subject's face to be properly exposed, while allowing the sand in the foreground to be overexposed.
While matrix metering systems, and the other metering systems discussed above, are moderately successful in a variety of common amateur snapshot situations, they are not sufficient to handle demanding, complex, and unusual lighting situations. Weddings, where people are often wearing high contrast black and white clothing, or theatrical performances, where there can be rapidly changing high contrast lighting, are a few examples of demanding and complex lighting situations. In these demanding lighting situations existing exposure metering systems typically do not provide consistently properly exposed images and frequently produce an image where the main subject of interest is either underexposed or overexposed.
Often an image captured by a DSLR may contain both properly exposed and underexposed regions. For DSLRs, moderate to significant amounts of underexposure can be compensated for in post-processing. Post-processing is where an image that has been captured and saved by the DSLR is uploaded to a computer and edited using digital image editing software. The underexposed regions of an image have lower signal-to-noise ratios (SNRs) than properly-exposed regions, which results in a lower image quality in those regions. However, using post-processing software, usable final images can often be obtained from underexposed images captured in the DSLR.
Where a significantly underexposed region of an image often contains useful information, a significantly overexposed region typically does not. In DSLRs, the significantly overexposed region of an image is recorded as pure white; where the Red color level, Blue color level, an Green color level are all equal to 255 on the RGB digital color table. In such an overexposed region, the overexposed region of the image will typically appear as a uniform area of maximum brightness, losing image detail and useful information. Thus, no image detail can be recovered in post-processing from the significantly overexposed region of the image. The significantly overexposed regions of an image recorded by a DSLR are often referred to by photographers as “blown-out highlights” or “blown highlights,” where a highlight is a bright region of an image and the blown highlight is an overexposed portion thereof.
Blown highlights may be acceptable in some images where the highlights are not of primary interest in the photograph. For example, specular highlights resulting from reflections of light sources in metallic objects can be blown, without detracting from the main subject of the image. However, when the main regions of interest in the photograph are the brightest areas of the composition, blowing the highlights can mean a catastrophic failure of an image being produced. Such situations can include bridal portraits, where the highlights include the fine details of the bride's dress, and theatrical photography, where the subjects' faces are brightly lit against a dark background.